1. Field of the Invention
The present invention relates to a digital transmission system, more particularly to an orthogonal frequency division multiplexing (OFDM) receiver, a semiconductor integrated circuit, and an OFDM method for receiving a signal, capable of detecting interference residing in a transmission line.
2. Description of the Related Art
The OFDM system is becoming a mainstream system for digital transmission systems for audio and video signals. In the OFDM system, data are allocated to a plurality of carriers which are orthogonal to one another. A transmitter performs inverse fast Fourier transform (IFFT) to a transmitting signal. A receiver demodulates a received signal by fast Fourier transform (FFT). For each carrier, an arbitrary modulation method such as quadrature amplitude modulation (QAM) transmission with synchronous detection, or differential detection can be applied. In the case of the synchronous detection, pilot signals are inserted periodically to the transmitting signal. The receiver performs amplitude equalization and phase equalization by finding errors based on the pilot signals in the receiving signal. In the case of differential detection, the receiving signal is demodulated by performing differential encoding between symbols without recovering the carriers.
However, in a transmission line, interference such as fading due to multipath, co-channel interference attributable to a carrier wave for analog television broadcasting, spurious interference in the transmission line, or the like exists. In the case of synchronous detection, the received signal cannot be accurately demodulated when there is interference with the pilot signals. For example, as the related art to detect interference of the pilot signals, the technique has been proposed to integrate, in a time direction, every used carrier for the pilot signals in the receiving signal. In other related art, the technique of detecting interference from fluctuations of the pilot signals in the time direction has been proposed.
The number of carriers used in ground wave digital broadcasting is far greater than the number of carriers used in a mobile communication device, a wireless local area network (LAN), and the like. According to the first discussed related art, a scale of the circuits required for detection of the interference is increased in proportion to an increase in the number of the used carriers. According to the second discussed related art, it is not possible to detect fluctuations in a frequency direction just by detecting the interference from the time jitter of the pilot signals. When the fluctuations in the frequency direction are not detectable, it is difficult to detect precisely the interference.